1. Field of the Invention
The present invention relates to a method for reducing the flow of fluids through a permeable formation at distances beyond the near wellbore region around wells, to achieve more uniform flow of fluids in subterranean rocks. More particularly, a method for in situ crosslinking of water-soluble polymers is provided which applies to processes in which fluids are injected or produced through wells for recovery of hydrocarbons or other minerals and for any other purpose involving flow of fluids.
2. Discussion of Related Art
In an effort to recover greater amounts of oil by injection of water or other fluids into wells, the petroleum industry has expended considerable effort in developing a variety of methods to achieve a more uniform flow of fluids through subterranean formations. Non-uniform flow, caused largely by variations of permeability of rock, leads to lower recovery of oil and increased cost of injection processes. Often the non-uniform flow arises from the presence of high permeability channels, which may be caused by varying depositional environments when the subterranean rock is formed, by natural or induced fractures or by dissolution of the rock to form vugs. In either case, there is a need to increase the flow resistance and thereby decrease the flow through the higher permeability zones.
A method widely investigated in the petroleum industry for increasing the flow resistance of higher permeability zones is the use of water-soluble polymers. It has been found that these polymers can be made more effective as permeability-reduction agents if the polymer molecules are crosslinked, which causes them to form a very viscous solution or a gel. Commonly used polymers, such as polyacrylamide, contain negatively charged ionic groups, so crosslinking by ionic forces can be achieved by charged polyvalent metal ions, preferably with a positive charge of 3. The polymers cannot be injected into many rocks if the cross-linking occurs before the polymer solution is injected, because the viscosity of the solution becomes too high; therefore, means are sought for providing a delayed crosslinking.
An early method for delayed crosslinking of polymers injected into wells was disclosed in U.S. Pat. No. 3,762,476. Injection of alternate slugs of polymer solution and metal ions which are retarded in their availability for crosslinking was proposed. The polymer and metal ions mix by flow in the rock to form a gel or viscous solution. Later, the same inventor proposed in U.S. Pat. No. 3,833,061 that, for oil-wet rocks, an oxidizing agent such as sodium dichromate solution be injected and flowed through the reservoir first, to remove any oil film from the surface of the rock, and be followed by a polymer solution, then by a complexing solution of metal ions retarded in their crosslinking action, then followed by a second polymer solution. The polymer and metal ions mix by flow in the rock to form a gel or viscous solution.
An early proposal for use of an oxidation-reduction reaction to achieve delayed crosslinking of polymers in a subterranean rock was in U.S. Pat. 3,785,437. This system utilized either an oxidizing or a reducing chemical in a polymer solution. Alternate slugs of the polymer solution are injected with a solution of the opposite type chemical to that in the polymer solution. The oxidation-reduction reaction occurs between the two chemicals in the alternating slugs of liquid by mixing of the liquids in the rock, causing the formation of a multivalent metal ion which can crosslink the polymer. The crosslinking is said to be possible out in the formation away from the treating well. Oxidizing agents suggested include chromates and reducing agents include sulfides and su fites.
U.S. Pat. No. 3,949,811 proposed alternate slugs of polymer solution with brine containing high concentrations of divalent or trivalent ions such as iron, aluminum, magnesium and chromium. Use of different degrees of ionic content of the polymer was disclosed. No provision was made for delaying the activity of the metal ion other than by the mixing of fluids by flow through the rock and no oxidation-reduction reactions occur.
U.S. Pat. No. 4,343,363 addresses the problem of plugging of the injection face of the rock in a well by polymers. An acidic aqueous solution is injected which contains an oxidizing agent such as sodium dichromate which degrades the polymer on the face of the rock, thereby forming a metal ion which can crosslink a polymer; a spacer is injected and followed with more polymer solution, which will plug the face of the rock again; then the process is repeated.
Articles reviewing the use of crosslinked polymers for modifying flow patterns in subterranean rocks have been recently published. Paper SPE/DOE 17331, "Evaluation of Commercial Crosslinked Polyacrylamide Gel Systems for Injection Profile Modification," by J.D. Purkaple and L.E. Summers describes detailed tests of 15 commercially available polyacrylamide polymers crosslinked with chromium ion formed in an oxidationreduction reaction.
The use of water-soluble polymers to decrease flow in zones which are causing inefficient displacement processes has significant utility, but improved methods are needed to allow crosslinking and gel formation far removed from wells, where pressure gradients are lower. Gel formation in a volume of the reservoir where pressure gradients are lower will allow the gel to block flow more effectively because the gel will not be displaced by high pressure gradients that exist near i0 wells. The method of this invention allows placement of the gel over a large portion of the area swept by the injection fluids. This will result in permeability reduction of high permeability zones and improvement of the sweep efficiency throughout the reservoir.